1. Field of the Invention
The present invention relates to a stacked piezoelectric element and a producing method therefor.
2. Related Background Art
Conventionally there has been proposed a stacked piezoelectric element in which an electro-mechanical energy converting material such as piezoelectric ceramics having electro-mechanical energy converting function and an electrode material are alternately stacked. Such stacked piezoelectric element, in comparison for example with plate like single piezoelectric ceramics of a same thickness, can provide a larger distortion for deforming or a larger generated force with a lower applied voltage, and is therefore investigated and employed for use in the driving portion of a vibration element constituting a vibration driving device such as a piezoelectric actuator or a vibration wave motor.
The stacked piezoelectric element is produced principally in the following two methods.
The first producing method consists of forming electrode layers on both surfaces of a sintered single piezoelectric ceramic plate, stacking a plurality of such ceramic plate and adhering, for example with adhesive material, the ceramic plates.
The second producing method is an integral sintering method consisting of superposing and thermally pressing layers of unsintered sheet-shaped molded member (green sheet) containing piezoelectric ceramics and an organic binder and unsintered layers of electrode paste and sintering thus superposed layers.
For connecting the electrode layers in thus produced stacked piezoelectric element with the outside, there is proposed a method of forming electric connection between the layers by a through hole (via hole) electrode, which is a penetrating electrode obtained by forming a through hole in each piezoelectric ceramic layer and filling such through hole with an electrode material.
Particularly in the second producing method for the stacked piezoelectric element, the through hole (via hole) electrode is obtained by forming a hole in the green sheet of the piezoelectric ceramics and filling the holes with conductive paste before stacking, and then sintering the green sheets after stacking.
A stacked piezoelectric element, in which such through hole (via hole) electrode is exposed on the surface layer of the stacked piezoelectric element and is used as the electric conductive means to the external circuit such as a printed wiring board, is proposed for example in the Japanese Patent Application Laid-open No. 8-213664.
When such proposed stacked piezoelectric element is employed in rod or Langevin type of a vibration wave actuator or motor, the reference shows that the smoothness of the upper and lower surfaces of the element affects the mechanical quality coefficient (Qm) of the entire device.
It is therefore proposed to apply surface processing (lapping, polishing, grinding etc.) to the upper and lower surfaces of the stacked piezoelectric element after sintering, thereby obtaining the element of high flatness.
The through hole (via hole) electrode of the stacked piezoelectric element may be provided at a position arbitrarily selected for forming electrical conduction between the layers.
For example in FIG. 6A, in a piezoelectric ceramic 2-1 of a first layer, constituting the surface layer of the stacked piezoelectric element 1, a through hole electrode 4-1 is formed in a position suitable for external connection.
In a piezoelectric ceramic 2-2 of a second layer, a through hole electrode 4-2 is provided in a position suitable for connecting the piezoelectric ceramic 2-1 of the first layer to a piezoelectric element 2-3 of a third layer so as to be made an electrical connection by an electrode film 3-2.
In the third and lower layers, a through hole electrode 4-3 and further through hole electrodes are provided in positions having relatively little contribution to the mechanical force generation, in order to achieve effective force generation of the stacked piezoelectric element 1.
However, the surface processing on the upper and lower surfaces of the stacked piezoelectric element of the above-described configuration has revealed the necessity for further improvement as will be explained in the following.
In the piezoelectric ceramics 2 constituting the stacked piezoelectric element 1 and the conductive member constituting the through hole electrodes 4 for the different layers, the green sheet and the conductive paste containing the conductive material show mutually different contraction rates at the sintering operation, because of the characteristics of each material and the different mixing ratios of the organic binder present prior to the sintering.
For this reason, there is generated a residual stress in a hatched area .alpha. shown in FIG. 6A. A defect from the falling off is easily generated in such area .alpha., and it is confirmed that, if the smoothing surface processing is applied to the upper and lower surfaces of such stacked piezoelectric element 1, a part of the piezoelectric ceramic drops off in an area .beta. shown in FIG. 6B.
Such dropping off of the piezoelectric ceramic exposes the through hole electrode 4-2, leading to a drawback such as short circuiting with the wiring on the printing wiring board.
On the other hand, the electrode material employed in the above-described process is generally composed of a precious metal (platinum, palladium, silver etc.) having high melting temperature or a mixture thereof because it has to be sintered together with the piezoelectric ceramic material. The electrode material is most commonly composed of a mixture of silver and palladium with a weight ratio of 5:5 to 8:2 though it is dependent also on the sintering temperature of the piezoelectric ceramic.
Such precious metals are expensive and constitute the largest proportion in the material cost of the stacked piezoelectric element. For this reason, the electrode layers are formed as thin as possible within the manufacturable range of the stacked piezoelectric element and within the acceptable performance range thereof, by the improvement in the electrode paste consisting of the electrode material, organic binder, solvent and other additives and the layer forming method such as screen printing.
The piezoelectric element thus formed is then subjected to a polarization process for enabling elongation and contraction for the actual use. The uppermost surface electrode layer is used as a contact electrode in such polarization process but is removed after the polarization process, thereby enabling connection of the individual through hole (through hole filled with conductive material being often called a through hole electrode or a via hole) exposed on the surface with the driving circuit.
Prior to the actual use, the stacked piezoelectric element is subjected to the polarization process of applying a voltage to the piezoelectric ceramic layers. This process is executed by applying a voltage of 3 to 1 kV/mm for a period of 30 to 60 minutes under a high temperature (80.degree. C. to 200.degree. C.).
In this process, the current generated after the voltage application becomes very large in case of the stacked piezoelectric element because of the significantly larger electrostatic capacitance in comparison with the conventional single-plate piezoelectric element. Consequently a large current is generated in the thin electrode layers of the stacked piezoelectric element, particularly in the connecting portion between the surface electrode layer in direct contact with the DC power source for the polarization process and the conductive electrode connecting the surface electrode to the internal electrode layers, whereby sparks are often generated to induce fused breakage of the connecting portion, thus inhibiting the polarization process or eventually resulting in the destruction of the element by the shock of the sparks.
The stacked piezoelectric element utilizes, in the conductive electrode for connecting the internal electrode layers, the through hole commonly employed in the printed circuit board, and has an internal electrode layer (to be explained later) having a circuit wiring function in addition to the surface electrode layer in direct contact with the DC power source. Such wiring electrode layer also causes sparks or fused breakage at the connecting portion with the conductive electrode, leading to the destruction of the device.
FIGS. 13A and 13B illustrate a stacked piezoelectric element described in the Japanese Patent Application Laid-open No. 8-213664. The stacked piezoelectric element 11 in FIG. 13A is composed, as shown in FIG. 13B, of n piezoelectric layers (piezoelectric ceramic layers) 14 (14-1 to 14-n). In the stacked piezoelectric element 11, the second and subsequent piezoelectric layers (14-2 to 14-n) are respectively provided with electrode layers 13 (13-2 to 13-n) for example of 4-divided configuration, and the electrode layers requiring mutual conduction are connected by through holes 12 penetrating through the piezoelectric layer.
More specifically, each of the first to (n-1)th piezoelectric layers 14-1 to 14-(n-1) is provided with eight through holes, and the first through hole 12-1-1 of the first piezoelectric layer 14-1 is connected to the first electrode 13-2-1 of the second electrode layer while the third through hole 12-1-3 of the first piezoelectric layer 14-1 is connected to the third through hole 12-2-3 formed in the second piezoelectric layer 14-2 and to the second electrode of the third electrode layer on the third piezoelectric layer 14-3.
Subsequently the first through hole formed in each piezoelectric layer is similarly connected to every even numbered layer, to the through hole 12-(n-2) of the third piezoelectric layer from the bottom and to the first electrode layer 13-(n-1)-1 on the second piezoelectric layer 14-(n-1) from the bottom. The third through hole in the third and subsequent piezoelectric layers is connected similarly to every odd numbered layer, and to the second electrode formed on the lowermost piezoelectric layer 14-n.